Alloy purification process



United States Patent 3,325,278 ALLOY PURIFICATION PROCESS Melvin L.McClellan, Grand Island, N.Y., assignor to Union Carbide Corporation, acorporation of New York No Drawing. Filed May 7, 1964, Ser. No. 365,7782 Claims. (Cl. 7560) The present invention relates to the purificationof alloys. More particularly, the present invention relates to a processfor the removal of impurities from molten metals by the oxidation of theimpurities.

Many metals and alloys, by virtue of their method of manufacture,contain minor amounts of impurities which detrimentally affect theirperformance in. their intended use. 1

For example, ferrosilicon which is used in the treatment of steel, oftencontains small percentages, e.g. up to 2 percent each of aluminum andcalcium.

In many instances, for example in the production of special electricalsteels containing silicon the presence of such amounts of aluminum andcalcium is highly undesirable since these elements interfere with theelectrical properties of the steel.

It has been proposed to remove these impurities by melting the alloyunder a high silica slag or holding the alloy molten under such a slagin an arc furnace. However .this type of procedure is costly andrequires relatively expensive equipment.

It has also been proposed to treat aluminum and calcium containingferrosilicon with gaseous oxygen while in the molten state to oxidizeand thus remove these materials.

This approach has not been entirely successful due .to the fact that asthe aluminum and/or calcium contents are lowered, substantial oxidationof silicon begins and relatively large amounts of silicon, e.g. up to 5percent and more are lost.

Moreover, as the silicon is oxidized, it forms a fluid slag on thesurface of the molten metal which has the effect of retarding furtheroxidation of the calcium and aluminum impurities and results in theinefiicient usage of oxygen and also causes slag separation problems.

It is therefore an object of the present invention to provide animproved process for removing impurities from molten metal through theuse of gaseous oxygen.

Other objects will be apparent from the following description andclaims.

A process in accordance with the present invention comprises providing amolten bath of the metal to be purified; contacting the surface of themolten metal with a stream of gaseous oxygen; and introducing inert gasinto the lower portion of the molten metal bath to provide agitationthereof to thereby promote oxidation of the oxidizable impuritiestherein.

In the practice of the present invention impurity-containing metal forexample ferrosilicon (74 to 76 percent Si) containing 0.45 percent Aland 0.20 percent Ca is provided in the molten state.

Molten ferrosilicon or other metal can be concurrently obtained by.tapping from the furnace in which it is produced into a ladle orsimilar device.

With the metal bath at a convenient temperature, e.g. 1350 to 1700 C. astream of oxygen is directed at the surface of the molten metal whileconcurrently an inert gas, e.g. argon is introduced into the lowerportion of the bath. The argon introduction can suitably be accomplishedby means of a tuyere located in the bottom of the ladle containing themolten ferrosilicon. It has been found that the rate of argonintroduction is preferably no greater than the rate of oxygen flow, inorder to permit effective removal of aluminum and calcium. For bestresults, the oxygen flow should be about 1 to 2 times the rate of argonflow.

Also, the oxygen flow should be limited to a value which avoids theoxidation of metal other than the impurities to be removed. That is tosay, the oxygen rate is limited so that there is no substantial slagformation on the surface of the metal.

By way of a more specific description of an embodiment of thepresentinvention, a batch of molten ferrosilicon (75.8% Si, 0.20% Ca,0.45% Al bal. Fe) is placed in a If a slag begins to form, the oxygenflow rate is suit-.

ably decreased to avoid further oxidation of silicon. It has been foundthat with the equipment and materials aforedescribed an oxygen flow rateof about 360 c.f.h. and an argon flow rate of about 225 c.f.h. canprovide a reduction of the aluminium and calcium content to below about25% and .06% respectively, without any significant loss of silicon, i.e.the silicon loss is less than 1 percent.

It is to be noted that in the present invention, the inert gas cannot bemerely mixed with the oxygen since under these circumstances onlydilution is achieved and little if any improvement in impurity removalis obtained.

In order to determine the effectiveness of the deoxidizing process ofthe present invention, melts of percent ferrosilicon were produced underlime-alumina slag. The slag was frozen in place and the metal was pouredinto a ladle having a 30 inch ID. and equipped with a tuyere inch indiameter centrally located at the bottom. The depth of the alloy in eachcase was about 22 inches. Argon was introduced through the tuyere andbubbled through the melt and a stream of oxygen was blow onto thesurface of the molten metal through a water cooled lance. This treatmentwas continued in each instance until the aluminum content of the alloywas decreased to about 1.0 percent and the calcium content to about 0.5percent. The calcium and aluminum contents of the starting alloy wereabout 1.7 percent and 1.6 percent, respectively.

The results obtained are shown in Table I.

TABLE I Weight of O2 Argon Slag Temp. at Starting Rate, Rate, Time,Forma- End of Alloy, lbs. c.f.h. c.i.h. minutes tion Treatpient,

I 1,590 720 225 15 Heavy 2,880 II, 1,750 360 225 25 None 2, 685

1 Silicon oxidation.

treatment of manganese, chromium, and iron base alloys.

What is claimed is:

1. A process for removing aluminum and calcium impurities fromferrosilicon which comprises:

(1) providing a molten bath of ferrosilicon (2) contacting the surfaceof the molten metal with a stream of gaseous oxygen and (3) introducinginert gas into the lower portion of the molten ferrosilicon bath .toprovide agitation thereof to thereby promote oxidation of the calciumand aluminum impurities therein.

2. A process for removing aluminum and calcium impurities fromferrosilicon which comprises:

(1) providing a molten bath of ferrosilicon at a temperature in therange of about 1350 C. to about 1700 C.; l

(2) contacting the surface of the molten ferrosilicon with gaseousoxygen by directing a stream of oxygen at the surface of the moltenferrosilicon;

(3) introducing inert gas into the molten ferrosilicon below the surfaceof the molten bath and at the lower portion thereof, the rate of flow ofinert gas being sufficient to cause vigorous agitation of the moltenbath by bubbles of inert gas; and

(4) adjusting the rate of How of oxygen and inert gas so that the oxygenflow rate is between about 1 and 2 times the inert gas flow rate and sothat there is no substantial formation of slag on the surface of themolten ferrosilicon.

References Cited UNITED STATES PATENTS 1,968,917 8/ 1934 Soldatoff 75602,866,701 12/1958 Strauss 7560 3,169,058 2/1965 Nelson 75-59 3,201,2268/1965 Spolders et al 7560 FOREIGN PATENTS 872,995 7/1196 1 GreatBritain. 878,386 9/ 1961 Great Britain.

BENJAMIN HENKIN, Primary Examiner.

1. A PROCESS FOR REMOVING ALUMINUM AND CALCIUM IMPURITIES FROMFERROSILICON WHICH COMPRISES: (1) PROVIDING A MOLTEN BATH OFFERROSILICON (2) CONTACTING THE SURFACE OF THE MOLTEN METAL WITH ASTREAM OF GASEOUS OXYGEN AND (3) INTRODUCING INERT GAS INTO THE LOWERPORTION OF THE MOLTEN FERROSILICON BATH TO PROVIDE AGITATION THEREOF TOTHEREBY PROMOTE OXIDATION OF THE CALCIUM AND ALUMINUM IMPURITIESTHEREIN.